In an image forming apparatus such as an electrophotographic copier, after the circumferential surface of a photoreceptor drum has been uniformly charged by a charging unit, image exposure is carried out on the circumferential surface of the photoreceptor drum so as to electrostatically form a latent image. This latent image is developed by a developing unit to be visualized. In this way, a toner image is formed. The toner image formed on the circumferential surface of the photoreceptor drum is transferred by a physical means onto a recording paper conveyed in timed relation with the formation of the toner image. The recording paper onto which the toner image has been transferred is separated from the photoreceptor drum surface, and conveyed to a fixing unit so that the toner image is fixed. After that, the recording paper is discharged outside of the image forming apparatus.
In the process of transfer in which the toner image formed on the circumferential surface of the photoreceptor drum is transferred onto a recording sheet, the toner image is charged so that it can be a polarity reverse to that of the toner image by a transfer unit that discharges behind the recording paper. By the action of the charge given by the transfer unit, the toner image is transferred onto the recording sheet. After that, a high AC voltage is impressed upon the recording sheet so that the recording sheet is neutralized and separated from the photoreceptor drum surface. However, it is difficult to positively ensure the transfer properties of the toner image and the separation properties of the recording sheet. Especially when the diameter of the photoreceptor drum is large, it becomes difficult to separate the recording sheet from the photoreceptor drum. When the recording sheet adheres onto the photoreceptor drum surface in the manner described above, paper jam tends to occur.
In order to take measures against the occurrence of jam, a sheet wrapping detection apparatus is conventionally provided, which detects whether or not a recording sheet has been separated from the surface of a photoreceptor drum.
With reference to FIG. 4, a conventional example of the aforesaid sheet wrapping detection apparatus will be explained as follows.
In FIG. 4, a reflection type photosensor 30 for detecting sheet wrapping is provided in the periphery of a photoreceptor drum 10 in the downstream of a transfer unit with respect to the direction of rotation of the photoreceptor drum 10. As illustrated in FIG. 4, this photosensor 30 includes: a light emitting diode 31, which is a light emitting element, being provided so that it can irradiate a beam of light in a direction perpendicular to the surface of the photoreceptor drum 10; and a phototransistor 32, which is a light receiving element, being provided so that it receives a beam of light reflected on the surface of the photoreceptor drum 10 after the beam of light has been emitted from the light emitting diode 31, wherein the reflection angle is .theta. on the photoreceptor drum surface. The photosensor 30 is controlled by a control unit 40 having a microcomputer.
The light emitting diode 31 emits light in accordance with a light emitting signal outputted from the control unit 40 in a predetermined timed relation, and the beam of emitted light is perpendicularly irradiated on the surface of the photoreceptor drum 10. Reflected light is received by the phototransistor 32. In the case of a sheet of paper on which a large amount of light is irregularly reflected, an amount of photoreception of the phototransistor 32 is large. On the other hand, in the case of the photoreceptor 10 on which a large amount of light is regularly reflected and a small amount of light is irregularly reflected, an amount of photoreception of the phototransistor 32 is small. In some cases, developed toner is left on the surface of the photoreceptor drum 10. In this case, a beam of light reflected on the residual toner is received by the phototransistor 32, and the level of received light becomes approximately the same as that of received light in the case of photoreceptor drum 10.
Accordingly, the operation is conducted in the following manner:
As shown in FIG. 5, the photoreception level of the photoreceptor drum and that of the toner are expressed by one-dotted chain lines, and the photoreception level of a recording sheet is expressed by a solid line. According to the difference between both photoreception levels, a threshold value S is set between both photoreception levels. This threshold value S and a photoreception output of the phototransistor 32 are compared by the control unit 40. In the case where the photoreception level is lower than the threshold value S, it is judged that the beam of light is reflected on the photoreceptor drum 10 or toner. Therefore, it is judged that no recording sheet is wrapped around the photoreceptor drum 10. In the case where the photoreception level is higher than the threshold value S, it is judged that the beam of light is reflected on a sheet of paper. Therefore, it is judged that a recording sheet is wrapped around the photoreceptor drum 10, and the apparatus is automatically stopped. In this connection, as illustrated by solid lines in FIG. 5, the photoreception levels of the photoreceptor drum 10, of toner, and of the recording sheet are deviated in accordance with the apparatus.
In the case where the image forming apparatus was stopped according to the result of judgment that a recording sheet had wrapped around the photoreceptor drum, it is necessary to check the release of the wrapped recording sheet before the start of a warm-up operation of the apparatus. Conventionally, in order to check the release of the wrapped recording paper, a comparison is made between the threshold value used when the apparatus was stopped, and the output value outputted from the photosensor 30 when the apparatus is restarted.
However, as shown in FIGS. 6 and 7, the conventional photosensor 30 is disposed in the apparatus so that a beam of light L emitted from the light emitting diode 31 meets at right angles with the axis A of the photoreceptor drum 10.
In this case, the circumferential surface of the photoreceptor drum 10 is cleaned by a blade to remove the residual toner from the surface after a toner image has been transferred on to a recording sheet. In the process of cleaning, the circumferential surface of the photoreceptor drum 10 is slightly damaged, that is, minute scratches are caused on the surface in the circumferential direction. Also, when the photoreceptor drum is manufactured, the drum surface is subjected to brushing in the circumferential direction. Therefore, minute scratches are caused on the surface in some cases. Due the scratches, beams of light are irregularly reflected on the surface of the photoreceptor drum 10. An amount of light irregularly reflected in the axial direction of the drum is larger than that irregularly reflected in the circumferential direction.
As a result of the foregoing, in the conventional photosensor arrangement, the photoreception level of the photosensor 30 is greatly deviated due to the reflected light on the photoreceptor drum 10 as shown in FIG. 5. As an amount of irregularly reflected light is increased, a difference between the photoreception level with respect to a recording sheet and that with respect to a photoreceptor drum becomes small. Therefore, it is difficult to appropriately set a threshold value so as to discriminate both photoreception levels, which is a problem encountered in the conventional photosensor arrangement.
Also, the output values of the individual photosensors 30 are deviated. Further, the output values are deviated when the photosensors 30 are stained with toner, or the environmental temperature is changed. Further, an amount of photoreception of the photosensor 30 varies according to the type of recording sheets. Due to the foregoing, the following problem may be encountered in the conventional device: In the conventional device, the threshold value is fixed. Therefore, erroneous detection of sheet wrapping tends to occur.
Unlike monochromatic printing in which black toner, the photoreception level of which is greatly different from that of a recording sheet, is used, in the case of color printing in which a plurality of toners such as yellow toner are used, the photoreception level of the photosensor 30 with respect to yellow toner is close to that with respect to recording sheets. In this case, there is a possibility of erroneous detection.
Furthermore, the characteristics of the photosensor 30 are varied when the environmental temperature is changed. Therefore, the characteristics of the photosensor 30 at the time of detection of release of sheet wrapping is different from those at the time of detection of sheet wrapping. In addition to that, the threshold value is fixed. For that reason, an erroneous detection is caused in the following manner:
Although a wrapped recording sheet has been released, the result of detection is that the recording sheet still wraps around the photoreceptor drum.
Especially, there is a high possibility of the erroneous detection in the case where a toner such as yellow toner, the photoreception level of the photosensor 30 of which is close to that of recording sheet, is deposited on the photoreceptor drum surface. In the erroneous detection, although a wrapped recording sheet has been released, the result of detection is that the recording sheet still wraps around the photoreceptor drum. There is also a possibility that while the recording sheet still wraps around the photoreceptor, the result of detection is that the recording sheet is released.
In view of the aforesaid problems, the present invention has been achieved. It is an object of the present invention to provide a sheet wrapping detection apparatus for use in an image forming apparatus in which the arrangement of the optical detection means is changed, so that a difference between the photoreception level with respect to recording sheets and that with respect to the photoreceptor drum can be made large, and the deviation of the photoreception level of the photoreceptor drum is small and the threshold value can be easily set, and as a result the occurrence of erroneous detection can be reduced.
In view of the aforesaid problems, the present invention has been achieved. It is an object of the present invention to provide a sheet wrapping detection apparatus for use in an image forming apparatus capable of positively detecting a sheet wrapping condition without being affected by the deviation of photoreception levels caused by the change of temperature and the stain of the optical detection means.
In view of the aforesaid problems, the present invention has been achieved. It is an object of the present invention to provide a sheet wrapping detection apparatus for use in an image forming apparatus in which the release of sheet wrapping can be reliably performed.